Long-period gratings can be written into photosensitive media of optical waveguides using amplitude masks that contain an array of windows for illuminating periodically spaced sections of the photosensitive media. The illuminated sections undergo a change in refractive index that supports coupling of selected wavelength bands between different propagating modes of the waveguides.
The periodic index variations of long-period gratings required to support forward propagating mode shifts along the lengths of optical waveguides have periods generally in the hundreds of microns (but can range from around 10 microns to 1000 microns). The period xe2x80x9cxcex9xe2x80x9d can be related to a difference between a propagating constant xe2x80x9cxcex201xe2x80x9d of the fundamental mode and a propagating constant xe2x80x9cxcex2cl(n)xe2x80x9d of the xe2x80x9cnthxe2x80x9d cladding mode as follows:             β      o1        -          β      cl              (        n        )              =            2      ⁢      π        Λ  
Since the periods xe2x80x9cxcex9xe2x80x9d are generally well beyond the wavelengths of the radiation used to write long period gratings, which is typically in the ultraviolet range, simple shadowing techniques can be used to produce the periodic intensity patterns required to write long-period gratings in photosensitive media. Typically, the photosensitive medium is irradiated by a field of actinic radiation (e.g., intense ultraviolet radiation) that is interrupted by an amplitude mask so that alternating portions of the field are either transmitted to the photosensitive medium or blocked from reaching the photosensitive medium.
Amplitude masks are commonly formed by metal or dielectric coatings laid down in patterns on transparent glass substrates. The coatings block unwanted light from reaching the photosensitive medium. Uncoated portions of the glass substrate framed by the coatings provide xe2x80x9cwindowsxe2x80x9d through which the desired intensity patterns reach the photosensitive medium. Some of the coatings block the unwanted radiation primarily by absorption and others of the coatings block the unwanted radiation primarily by reflection, although both processes occur to some extent in all coatings.
Either type of coating can be degraded by exposure to intense radiation. The absorptive coatings (typically a metal) are subject to ablation, melting, or heating sufficient to promote pealing from the glass. The reflective coatings (typically a dielectric) are expensive, difficult to manufacture, and can still be destroyed by intense radiation. Consequently, intensities are kept low, requiring unnecessarily long exposure times to achieve the desired refractive index change in the photosensitive medium.
The invention overcomes the durability problems of prior amplitude masks used for making long-period gratings by scattering, redirecting, or otherwise diverting alternating portions of the illuminating radiation instead of blocking unwanted radiation portions by absorption or reflection. The new amplitude mask transmits both the radiation intended to reach periodic segments of a photosensitive media and the radiation intended not to reach adjacent segments of the photosensitive media. Optical paths taken by the two transmissions are relatively modified, however, to produce the required pattern of illumination while reducing transformations of light energy into heat, thereby allowing long-period gratings to be written more quickly and at higher intensities.
One example of the new amplitude mask includes a base optic made of a material capable of transmitting actinic radiation for writing long-period gratings in a photosensitive medium of optical waveguides. A shadow-forming pattern along the base optic provides for illuminating periodically distributed segments of the photosensitive medium with the actinic radiation transmitted through the base optic. First transmissive portions of the shadow-forming pattern convey first portions of the actinic radiation to the periodically distributed segments of the photosensitive medium, and second transmissive portions relatively divert second portions of the actinic radiation away. from adjacent segments of the photosensitive medium.
The second transmissive portions can include (a) diffusers for scattering the second portions of the actinic radiation, (b) diffractors for diffracting the second portions of the actinic radiation, or (c) refractors for relatively bending the second portions of the actinic radiation away from adjacent segments of the photosensitive media. The diffusers generally diminish the concentrations of radiation reaching the adjacent segments of the photosensitive medium. The diffractors and refractors can also reduce the amount of radiation reaching the adjacent segments of the photosensitive medium by spreading the radiation or by redirecting the radiation to other intended locations. For example, the diffractors can be arranged with rulings oriented parallel to a optical axes of the waveguides for diffracting the second portions of the actinic radiation in a direction transverse to the waveguide axes. The refractors, which can include lenses or prisms, can also be arranged to bend light in the direction transverse to the waveguide axes.
The diffusers, diffractors, and refractors can be formed by etching, machining, or otherwise removing material from the base optic as well as by depositing, appending, or otherwise adding material or structure to the base optic. Shadow-forming patterns involving any or all of these transmissive diverting mechanisms can be formed on one or more surfaces of the base optic to produce singular or compound effects of diffusion, diffraction, or refraction.
For example, the base optic can be a glass plate with front and back surfaces through which both portions of the actinic radiation are transmitted, and either or both surfaces can be used to support a shadow-forming pattern. The base optic can also take the form of a prism having an entry surface through which both portions of the actinic radiation pass for entering the prism and one or more exit surfaces through which the two portions of the actinic radiation pass for exiting the prism. The two portions of the actinic radiation can pass through the same or different exit surfaces of the prism.
Normally, enough optical power is available for efficiently writing long-period gratings, and the new amplitude mask provides for conveying more of this available power to the photosensitive media. The new amplitude mask can also be arranged for use with lower power sources by making more efficient use of incident radiation. Instead of diverting the normally unwanted portion of the actinic radiation away from the photosensitive media, the new amplitude mask can be arranged with an array of transmissive mechanisms (e.g., refractors) to redirect this radiation to the otherwise illuminated segments of the photosensitive medium. Both the portions of the actinic radiation thereby contribute to writing grating patterns in the photosensitive medium.